This application concerns an electron beam memory system with improved high rate digital beam pulsing system.
It is a primary object of the invention described and claimed in my copending application Ser. No. 825,219 (now abandoned) to provide an electron beam memory system having an electron gun capable, for the first time, of developing high enough electron probe current densities to permit no-develop recording and small enough probe sizes to permit ultra-high density recording, yet of such low mass and compactness as to make feasible rapid random accessing of any area on the system's recording medium.
In the context of such a system, beam pulsing apparatus is needed which is compatible with and contributes to the achievement of these capabilities in the gun.
Electron beam digital pulsing systems are known which create an intermediate beam cross-over or focus between the source and ultimate probe (focus). A simple deflector is placed at the location of this intermediate focus and a suitable aperture is put into the succeeding lens. When the deflector is activated, the beam strikes the aperture support and is thereby attenuated. The beam, when undeflected, passes through the aperture unattenuated, the ultimate focus being unaffected.
The net result is that the recording beam can be turned off and on (pulsed) without being deflected sideways or becoming distorted. This is an effective way to pulse an electron beam, but requires a second lens--a constraint which all but rules it out as a feasible beam pulsing technique in a system of the type described in which gun compactness and low mass are of critical importance.
It is known in a scanning electron microscope system having a magnetic focus lens to add an auxiliary magnetic lens or an auxiliary winding on a main focus lens, and to apply, when working in a high resolution mode, a current pulse to the auxiliary winding or lens to selectively defocus the electron beam. See U.S. Pat. No. 3,919,500. The described technique is not used to apply pulse information. Rather, the defocused condition of the beam is used to develop a total-beam-current reference which is used to compensate the resultant data for instability in the beam current.
This scanning electron microscope technique would not be suitable for use in the electron beam memory system with which this invention is concerned, as the additional lens or winding would unacceptably add to the mass and bulk of the electron gun. Further, the inductance of the auxiliary lens or winding would unacceptably limit the achievable data recording rate.
U.S. Pat. No. 3,748,467 discloses a scanning electron microscope with an auxiliary lens which is switched between two current levels to create two fixed focal planes, the data gathered from which planes are displayed on separate cathode ray tubes. Again, the technique is not used for pulse information recording and would be similarly unacceptable for that purpose for the same reasons.
An auxiliary magnetic lens of the type called for in the afore-discussed patents is disclosed in U.S. Pat. No. 4,245,159.
U.S. Pat. No. 3,397,959 discloses a scanning electron microscope utilizing an auxiliary focus lens in an automatic focusing system for the microscope.